If the past few years have taught us anything, it’s that the boundaries of folk and country music - two genres steeped in decades’ worth of tradition - have proved much more pliable than we might once have expected.

After all, Bon Iver - who’s very nom de plume has become a byword for lovelorn, hushed tales of heartbreak spun over an acoustic guitar - took about as dramatic a left turn as you could’ve imagined with the glitchy, electronic stylings of 22, A Million, with the possible exception of him being unveiled as the new frontman of Cannibal Corpse. When Fleet Foxes emerged from the wilderness last year, they returned operating on a considerably grander and more experimental plane than six years previously. Grizzly Bear did likewise, themselves coming back from a period of radio silence.

The trend has begun to cut both ways, too; Justin Timberlake is about to release Man of the Woods, a record that has clearly borrowed greedily from the For Emma, Forever Ago aesthetic but that surely won’t stray too far from its creator’s penchant for electropop. Perhaps it’s always been this way; after all, as much as the folk rock of I’m Wide Awake, It’s Morning is what springs to mind when there’s mention of Bright Eyes, it’s worth remembering that their moody future-blues LP Digital Ash in a Digital Urn was released on the same day.

Amidst all of this sea change, though, First Aid Kit have stuck to their guns, even as their heroes have transformed around them. At the basic level, they’re still the same band that self-taped that cover of Fleet Foxes’ ‘Tiger Mountain Peasant Song’ in a Swedish forest a decade ago, even now that they’ve arguably eclipsed Robin Pecknold’s outfit in commercial terms. They’re still the same musicians that took so many of their early cues from the more straightforward Bright Eyes albums, even after they’ve toured with Conor Oberst and had their last two albums produced by Mike Mogis.

One of their more impressive tricks of the last few years has been not just turning up at Sweden’s Polar Music Prize ceremony to perform in honour of the given year’s recipient of the lifetime achievement award, but doing so with such power that said recipient is reduced to tears; this was true of Patti Smith, when the duo covered ‘Dancing Barefoot’ in 2011, and again of Emmylou Harris in 2015, when they played her the track they named after her. Paul Simon, too, looked choked up by the time they finished their own take on ‘America’ in 2012.

On top of that, First Aid Kit have worked their way, in under ten years, through most of the standard-issue bucket list for young bands - sold-out tours, major festival slots and, here in the UK, gold-certified records. You’d be forgiven for a double-take at the realisation that Johanna Söderberg , the older of the two sisters, is still only 27. They’ve handled their achievements with such poise, and carved out an accomplished musical blueprint so quickly, that they’ve earned the right to do things on their own terms.

That said, you wonder whether they’re getting to the point where the work is going to suffer from an abundance of caution. The stars all seemed to align so neatly for them on The Lion’s Roar in 2012 that you couldn’t blame them for remaining true to the same formula two years later with Stay Gold; in fact, when they did deviate from the framework by adding grand instrumental flourishes like string sections, they tended to blunt the sense of rolling drama rather than accentuate it.

Fans of both those albums are hardly likely to be left scratching their heads by Ruins, a handsomely-constructed set of alt-country songs that wisely pulls back from Stay Gold’s appetite for throwing the instrumental kitchen sink at the compositions. Instead, the embellishments to the tried-and-true formula are tasteful; swells of brass, a string section that’s employed with knowing restraint, and, on the countrified likes of ‘Postcard’, subtle slide guitar. Most crucially of all, that uncanny knack for making beautiful vocal harmonisation sound effortless that only ever seems to exist between siblings - ‘blood harmony’, to use the technical term - remains at the epicentre of these songs. It’s a quirk of genetics that’s surely played a major part in how swiftly the pair found their musical feet.

Still, though, it’s difficult to shake the feeling that they’re continuing to play it safe. Much of Ruins was apparently inspired by the breakup between Klara and her fiancé, as well as the strained relationship between the two sisters after two years of living in each others’ pockets on the road. Whenever they shoot for genuine lyrical darkness, though, it comes off asinine, particularly on opener ‘Rebel Heart’, a pretty milquetoast exercise in self-admonishment for bad habits. They’re far better at conveying wistful melancholy than they are genuine despair and for all of the idols that they now consider contemporaries or even friends, it’s the tragic likes of Townes Van Zandt and Gram Parsons that they’ll never catch up with, at least not until there’s the injection of real risk into their writing.

Ruins is a thoroughly pretty piece of work, lovingly presented. The question hanging over it, though, is how long First Aid Kit can get away with making revisions to the original model before the law of diminishing returns begins to kick in hard. Most of their fans are twice their age and there’s no reason to think that they won’t be charmed by these new songs, and to continue to cater to that audience might be the height of the band’s ambition. If so, then fair play to them, especially given that they’ve already achieved far more than most do before they hit 30.

The frustration, though, is that we’ve had glimpses of something more; their surprisingly heavy live shows, with exhilarating classic rock covers thrown in, are a case in point, as is their incendiary delve into gender politics on last year’s single ‘You Are the Problem Here’. The duo have the tools to capture a wider public imagination than they are doing presently, and as lovely as Ruins is without them, First Aid Kit’s lack of edge is beginning to look less like prudent risk aversion and more like self-inflicted damage.